Type of non-aqueous electrolyte

ABSTRACT

This invention relates to non-aqueous electrolytes, in particular, a non-aqueous electrolyte for lithium-ion secondary batteries. The electrolyte comprises regular organic solvents and electrolyte saline. The special characteristics are: the electrolyte also comprises mixed additives, said mixed additives comprising at least one of those of compound group A, at least one of those of compound group B, and one of those of compound group C wherein: compound group A are selected from inorganic saline including Li2CO3, Li2SO4, Li2 SO3, LiNO3; compound group B are selected from vinylene carbonate, propylene carbonate; and compound group C are selected from ES, PS, DMS, DES, DMSO. The weight ratio can be (Group I).

CROSS REFERENCE

This application claims priority from a Chinese patent applicationentitled “A Type of Non-Aqueous Electrolyte” filed on Nov. 10, 2005,having a Chinese Application No. 200510101336.8. This application isincorporated herein by reference in its entirety.

FIELD OF INVENTION

This invention relates to non-aqueous electrolytes. In particular, itrelates to a type of non-aqueous electrolyte for lithium-ion secondarybatteries.

BACKGROUND

As the demand for lithium-ion batteries increases, the requirement forthe technology is higher as well, such that the batteries manufacturedcan have greater energy density and better electrochemical properties tomeet the market demand. Here, enhancing the capacity of the lithium-ionbatteries, their cycling performance, and the electrochemical propertiesat high and low temperatures is the trend for current and futuretechnical improvements of the batteries.

Adding additives to the electrolyte to improve the properties oflithium-ion secondary batteries has been a research emphasis during therecent years. Since the SEI membrane (formed in the reactions betweenactive matters and the electrolyte during the initial charging process)can suppress solvent from co-intercalating into the graphite and has nonegative effects on the transmission of Li⁺, the characteristics of theadditives are important factors in affecting the properties of abattery. Thus they have become an emphasis and focus for research.Currently, quite a few property-improving additives optimize the SEImembrane. For example, U.S. Patent 2004091786 discloses that by addingpropylene sulfite (PS), a stable SEI membrane can be formed on thesurface of the carbon negative electrode. This membrane will not breakat high temperatures, blocks the reaction between the electrolyte andthe negative electrode, restricts the creation of gases, and improvesproperties at high temperatures. A. Naji has discovered that by addingethyl sulfite (ES) in a propylene carbonate (PC) electrolyte system, theco-intercalation phenomenon of PC in graphite is suppressed (seeElectrochim. 2000, (145):1893). Chinese Patent CN1599120 discloses thatby adding ES, the cycle life of batteries is improved. In addition,inorganic membrane-forming additives including carbon dioxide and sulfurdioxide can improve certain aspects of the properties of batteries. Manydifferent researches show that by adding vinylene carbonate (VC), apassivation film can be formed on the surface of electrodes. Thereduction potential of VC is higher than that of the solvents includingethylene carbonatediethyl carbonate (EC), PC, diethyl carbonate (DEC),dimethyl carbonate (DMC), etc. Thus VC can be reduced first on a carbonnegative electrode and forms stable SEI membrane, thereby improving thestability of a battery in the cycle process, especially the stability athigh temperatures (see J. Electrochem. Soc., 2001, 148:1341˜1345). U.S.Patent 2004062995 discloses that by adding VC, a surface film can beformed, thereby preventing the dissolution of electrolyte and improvingthe cycle life.

Although by merely adding the above additives, the properties of thebatteries can be improved to a certain degree, but the effect is notvery distinctive. Often, a certain property is improved and the othersare not affected. Since the additives are often costly, and some of theadditives are used in a large amount when it is used by itself, thecosts are greatly increased. As the market and customers increase theirdemand for high capacity and usability properties in different operatingconditions, the industry is pushed for better products. Therefore, it isdistinctively important to find an additive with good overallproperties, or a combination of additives which can improve variousproperties of batteries. In addition, it is important that the cost ofthe additives must be within an acceptable range.

SUMMARY OF THE INVENTION

An object of this invention is to provide a type of non-aqueouselectrolyte which can improve the capacity, cycle properties, andelectrochemical properties of batteries;

Another object of this invention is to provide a type of non-aqueouselectrolyte which can improve the performance of batteries at high andlow temperatures.

Still another object of this invention is to provide a cost-effectivetype of non-aqueous electrolyte.

Briefly, the non-aqueous electrolyte for lithium batteries comprisesregular organic solvents and electrolyte saline. The specialcharacteristics are: it comprises mixed additives, where said mixedadditives comprise at least one of those of compound group A, at leastone of those of compound group B, and one of those of compound group C,where Compound group A is selected from inorganic saline includingLi₂CO₃, Li₂SO₄, Li₂SO₃, LiNO₃; Compound group B is selected fromvinylene carbonate (VC), propylene carbonate; and Compound group C isselected from ES, PS, dimethyl sulfone (DMS), diethyl sulfone (DES),Dimethyl Sulphoxide (DMSO). The preferred weight percentage of saidingredients of the mixed additives in the non-aqueous electrolyte ofthis invention are further specified.

An advantage of a battery having the non-aqueous electrolyte with theadditives of this invention is that the battery would have goodlow-temperature properties

An advantage of a battery having the non-aqueous electrolyte with theadditives of this invention is that the battery would have good cycleproperties.

Another advantage of a battery having the non-aqueous electrolyte withthe additives of this invention is that the battery would have highcapacity.

Yet another advantage of a battery having the non-aqueous electrolytewith the additives of this invention is that the battery would havelower swelling, where the amount of gases generated in the formation andcycle processes is greatly decreased.

DESCRIPTION OF FIGURES

The following are further descriptions of the preferred embodiments ofthe present invention with references to figures and examples of theirapplications.

FIG. 1 is a comparative chart showing the cycle retention of anembodiment of this invention and a comparative example.

FIG. 2 is a comparative chart showing the thickness of the embodiment ofthis invention and a comparative example in the cycle procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the presently preferred embodiments of this invention, thenon-aqueous electrolyte for lithium batteries comprises regular organicsolvents and electrolyte saline. The special characteristics are: itcomprises mixed additives, where said mixed additives comprise at leastone of those of compound group A, at least one of those of compoundgroup B, and one of those of compound group C.

Compound group A: selected from inorganic saline including Li₂CO₃ Li₂SO₄Li₂SO₃% LiNO₃;

Compound group B: vinylene carbonate (VC), propylene carbonate;

Compound group C: ES, PS, dimethyl sulfone (DMS), diethyl sulfone (DES),Dimethyl Sulphoxide (DMSO);

The weight percentage of said ingredients of the mixed additives in thenon-aqueous electrolyte:

compound group A: 0.1%-3.0%;

compound group B: 0.5%-4.0%;

compound group C, 1.0%-5.0%.

The preferred weight percentage of said ingredients of the mixedadditives in the non-aqueous electrolyte of this invention:

compound group A: 0.3%-2.0%;

compound group B: 1.0%-3.0%;

compound group C, 1.0%-3.0%.

The more preferred weight percentage of said ingredients of the mixedadditives in the non-aqueous electrolyte of this invention:

compound group A: 0.5%-1.0%;

compound group B: 1.0%-3.0%;

compound group C, 1.0%-2.0%

A battery having the non-aqueous electrolyte with the additives of thisinvention have the following advantages:

1. Good low-temperature properties, meaning at −10° C., discharging thebattery at 1 C, a battery having the additives has an initial capacityat 2.7V, about 10% higher than a battery without the additives.

2. Good cycle properties, meaning LP043450 batteries can have a residualcapacity percentage of 80% after 400 cycles.

3. High capacity, meaning after adding the additives of this invention,the discharge capacity at 1 C is about 10-30 mAh higher than those nothaving the additives.

4. Lower swelling, the amount of gases generated in the formation andcycle processes is greatly decreased.

The following is a detailed description of various embodiments of thisinvention. Comparative charts are provided to further illustrate thisinvention.

1. The non-aqueous electrolyte comprises (1) organic solvent, (2)electrolyte saline, and (3) mixed additives.

(1) Normally, the organic solvent series can be carbonate, carboxylicester, sulfurized carboxylic ester, or a random combination of ketone,sulfone, furan, and ether. The carbonate can be ring carbonate or chaincarbonate. The ring carbonate includes various frequently used organicsolvents, mainly including EC, PC, 1,3-dioxolane(DOL), butylenecarbonate(BC), and the ramification of the saline. The chain carbonateincludes various frequently used chain carbonate such as dimethylcarbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC),methyl acetate(MF), etc. and their ramification. The furan can betetrahydrofuran or its ramification. Currently the carbonate is mostthoroughly researched, and is relatively ideal choice for the organicsolvent. In the embodiment of this invention, the organic solvents are acombined system with a ration of: EC:EMC:DEC=2:2:1.

(2) Normally, the electrolyte saline can be selected randomly from oneor a mixture of the following including LiPF₆, LiAsF₆, LiClO₄, LiBF₄,LiCF₃CO₂, Li(CF₃CO₂)₂N, LiCF₃SO₃, Li(CF₃SO₂)₃, Li(CF₃SO₂)₂N etc. Thisembodiment uses LiPF₆ and a solution of 1.0 mol/L is created with theabove organic solvent.

(3) The mixed additives added to the non-aqueous electrolyte of thisinvention comprise at least one compound of Compound group A, at leastone compound of Compound group B, and at least one compound of Compoundgroup C:

Compound group A: inorganic lithium-salt selected from Li₂CO₃, Li₂SO₄,Li₂SO₃, LiNO₃;

Compound group B: VC, PC;

Compound group C: ES, PS, DMS, DES, DMSO.

2. There are three ways of mixing the above organic solvents,electrolyte saline, and the mixed additives. Here, in the exemplarymixed additives, the preferred Compound group A is Li₂CO₃, the preferredCompound group B is VC, and the preferred Compound group C is ES:

(1) First, the organic solvents are mixed well based on the requiredratio and put aside. Then Li₂CO₃, VC, and ES are mixed together, shakenwell and put aside for 0-24 hours before being added to the preparedmixed organic solvent. The mixture is sealed, taken out and fired in avacuum oven at 20-90° C. vacuum or directly baked for 2-24 hours. It isplaced in a glove box and the electrolytes are added. It is shaken andput aside for above 24 hours.

(2) First, the organic solvents are added, and then Li₂CO₃ is added. Themixture is sealed, taken out and fired in a vacuum oven at 20-90° C.vacuum or directly baked for 2-24 hours. Then the mixture is put in aglove box where VC and ES are added. The mixture is shaken well and putaside for over 24 hours.

(3) First, VC and ES are added to the organic solvents. Then the mixtureis put aside for 0-24 hours. Then Li₂CO₃ is added. The mixture issealed, taken out and fired in a vacuum oven at 20-90° C. vacuum ordirectly baked for 2-24 hours. Then LiPF₆ is added in the mixture in aglove box. The mixture is shaken well and put aside for over 24 hours.

All the above solvents, additives and electrolyte saline are added in aglove box. The effect of moisture and other matters should be carefullycontrolled. Method (2) above is preferred for the embodiments of thisinvention.

3. When the fluid is injected, the following methods can be used:

a. The fluid can be directly injected using a designed manual fillingmachine in a glove box, and the electrolyte fluid can be heated to atemperature of 20-80° C.

b. The fluid can be injected using an automatic filing machine of theassembly line.

c. The fluid can be injected using a manual filling machine in a glovebox. The electrolyte fluid is shaken well before injection and it can beheated to a temperature of 20-80° C.

d. The fluid can be injected using a manual filling machine in a glovebox while being stirred. The fluid can be stirred using amagnetic-powered stirrer, and it can be heated to a temperature of20-80° C.

Solution d is preferred embodiment of this invention.

By adjusting the combination and amount of the three groups of compoundsA, B and C in the additives, embodiments 1-10, as well as thecomparative examples can be obtained.

Embodiment 1

The non-aqueous electrolyte comprises mixed additives of 0.3% weightpercentage of solid inorganic saline Li₂CO₃, 1.0% of VC, and 1.5% of ES.

Embodiment 2

The non-aqueous electrolyte comprises mixed additives of 0.8% weightpercentage of solid inorganic saline Li₂CO₃, 2.0% of VC, and 1.5% of ES.

Embodiment 3

The non-aqueous electrolyte comprises mixed additives of 0.5% weightpercentage of solid inorganic saline Li₂CO₃, 3.0% of VC, and 3.0% of ES.

Embodiment 4

The non-aqueous electrolyte comprises mixed additives of 1.0% weightpercentage of solid inorganic saline Li₂CO₃, 1.0% of VC, and 2.0% of ES.

Embodiment 5

The non-aqueous electrolyte comprises mixed additives of 1.0% weightpercentage of solid inorganic saline Li₂CO₃, 2.0% of VC, and 5.0% of ES.

Embodiment 6

The non-aqueous electrolyte comprises mixed additives of 1.0% weightpercentage of solid inorganic saline Li₂CO₃, 4.0% of VC, and 1.0% of PS.

Embodiment 7

The non-aqueous electrolyte comprises mixed additives of 3.0% weightpercentage of solid inorganic saline Li₂CO₃, 0.5% of VC, and 3.0% of ES.

Embodiment 8

The non-aqueous electrolyte comprises mixed additives of 0.1% weightpercentage of solid inorganic saline Li₂CO₃, 1.0% of VC, and 2.0% of ES.

Embodiment 9

The non-aqueous electrolyte comprises mixed additives of 1.0% weightpercentage of solid inorganic saline Li₂CO₃, 0.5% of VC, and 1.0% of ES.

Embodiment 10

The non-aqueous electrolyte comprises mixed additives of 1.0% weightpercentage of solid inorganic saline Li₂CO₃, 2.0% of VC, and 1.5% of ES.

Comparative Example

The non-aqueous electrolyte of the comparative example does not containmixed additives.

The ingredients and contents of the embodiments and comparative examplesare shown in Chart 1.

CHART 1 Unit: (wt %) Compound Compound group A group B Compound group CLi₂CO₃ Li₂SO₃ VC ES PS Embodiment 1 0.3 1.0 2.0 Embodiment 2 0.8 2.0 1.5Embodiment 3 0.5 3.0 3.0 Embodiment 4 1.0 1.0 2.0 Embodiment 5 1.0 2.05.0 Embodiment 6 1.0 4.0 1.0 Embodiment 7 3.0 0.5 3.0 Embodiment 8 0.11.0 2.0 Embodiment 9 1.0 0.5 1.0 Embodiment 10 1.0 2.0 1.5 Comparative 00 0 Example

The following is the test result of the above embodiments andcomparative example.

1. The other elements of a battery

Positive Electrode Plate

95 wt % LiCoO₂, 3 wt % PVDF, and 2 wt % acetylene black are mixed. ThenN-methylpyrrolidone is further added to the mixture. The resultingmixture is fully stirred to formed an evenly disperse slurry. Using astretcher, the evenly disperse slurry is spread on both sides of analuminum foil of 18 μm. A positive electrode plate is made after thefoil is heated in a vacuum environment and cut to obtain the desiredsize.

Negative Electrode Plate

95 wt % graphite, 2 wt % dispersant, 3 wt % adhesive PVDF, and a certainamount of water are stirred well to form a slurry. Using a stretcher,the mixture is spread on both sides of an aluminum foil of 12 μm. Anegative electrode plate is made after the foil is heated in a vacuumenvironment and cut to obtain the desired size.

Membrane: 20 μm PE Tonen membrane.

Battery designed to LP043450, designed capacity is 1 C+800 mAh.

2. Tested Items

(1) Capacity Test

A. Tester: BS-9300 (R) secondary battery properties testing device;

B. Testing conditions: normal temperature, relative humidity: 25-85%;

C. Charging mode: constant current constant voltage to 4.2 V;

D. The capacity testing results of the batteries (see Chart 2).

CHART 2 (Unit: mAh) Capacity Capacity Capacity Capacity CapacityCapacity Average 1 2 3 4 5 6 Capacity Embodiment 1 812 815 824 834 825817 821.2 Embodiment 2 833 826 839 841 824 823 831 Embodiment 3 833 825818 821 824 830 825.2 Embodiment 4 832 819 835 829 828 825 828Embodiment 5 804 817 822 831 820 814 818 Embodiment 6 845 829 825 826819 840 830.7 Embodiment 7 818 824 814 829 822 812 819.8 Embodiment 8822 818 811 808.8 814 823 816.1 Embodiment 9 815 808 831 827 810 828819.8 Embodiment 10 825 820 832 818 827 820 823.7 Comparative 797 803814 808 811 798 805.2 Example

(2) Cycles at regular temperature

A. Testing instrument: BS-9300 (R) secondary battery properties testingdevice, vernier calipers, etc.

B. Testing conditions: normal temperature, relative humidity: 25-85%

C. Charging mode: constant current constant voltage to 4.2 V

D. Discharging mode: constant current constant voltage to 3.0 V

E. Charging and discharging current: 1 C

F. Recording method: recording each cycle capacity using computers, andmanually measure the thickness of the batteries every 100 cycles.

G. Test results of the cycle residue rate (see FIG. 1)

H. Test results of the thickness of the batteries in the formation andcycle processes (see FIG. 2)

(3) Discharging at Low Temperatures

A. Testing instrument: hygrothermostat

B. Testing conditions: −10° C., for 90 minutes

C. Discharging current: 1 C

D. Recording method: specific capacity at 3.1V, 3.0V, and 2.75V

E. Test results of discharging at low temperatures (see Chart 3)

CHART 3 −10° C., 1 C Discharging Electrolyte and 3.1 V/Initial 3.0V/Initial 2.75 V/Initial Conditions Capacity(%) Capacity (%) Capacity(%) Embodiment 1 55.2 57.4 61.5 Embodiment 2 64.1 66.7 68.3 Embodiment 358.2 61.5 66.1 Embodiment 4 57.4 61.4 64.3 Embodiment 5 52.5 55.6 58.7Embodiment 6 60.4 64.3 67.7 Embodiment 7 57.6 62.4 65.1 Embodiment 852.6 57.2 62.4 Embodiment 9 56.7 59.4 63.2 Embodiment 10 60.3 64.2 66.4Comparative 47.8 49.2 54.3 Example

3. Evaluation

As shown in Chart 2, in the embodiments, after the three mixed additivesare added, the capacity of the battery has substantially increased. Thehighest capacity sees an increase of 25 mAh, and the effect isdistinctive. The properties of embodiments 2, 3, 4, and 6 are especiallygood. The reason is that by adding Li₂CO₃ or other inorganic saline, theloss of Li in the process of membrane-forming is compensated, decreasingirreversible capacity. VC allows the SEI membrane to be dense withstrong structure.

Referring to FIG. 1, it is shown in the comparative figure of cycleresidue rates that, the capacity of the comparative example batterywithout the additives declines very quickly, to about 80% after over 200cycles. However, the embodiments have good cycle effects. Specially,embodiments 1, 2, 4, 6, and 10 have a cycle residue rate of 80% after400 cycles. It shows that by adding the mixed additives, the cycles havedistinctively improved.

Referring to FIG. 2, the thickness of the battery before formation ismeasured using vernier calipers. The thickens after 100 cycles and 250cycles is tested for the average of three points—the top, middle, andbottom of the battery. As shown in the figure, by adding the mixedadditives, the amount of gases in the formation decreases. Meanwhile,the swelling during the cycle process is curbed. That is mainly due tothe effect of VC and ES, which form a fine SEI membrane on the surfaceof the negative electrode, thereby reducing the amount of gasesgenerated by the co-intercalation of the solvents in the reactions ofthe solvents with the SEI membrane.

Referring to Chart 3, the battery is discharged at 1 C at −10° C. Eachembodiment has a discharging capacity at low temperatures better thanthat of the comparative example. Here, the 2.75V/initial capacity (%) ofembodiment 2 at −101 C is higher than that of the comparative examplewithout the mixed additives by about 14%. The effect on discharging atlow temperatures is distinctive.

Thus it is shown that the object of this invention can be achieved whenthe weight percentages of the ingredients of the mixed additives in thenon-aqueous electrolyte are:

compound group A: 0.1%-3.0%;

compound group B: 0.5%-4.0%;

compound group C, 1.0%-5.0%.

The preferred weight percentages are:

compound group A: 0.3%-2.0%;

compound group B: 1.0%-3.0%;

compound group C, 1.0%-3.0%.

The more preferred weight percentages are:

compound group A: 0.5%-1.0%;

compound group B: 1.0%-3.0%;

compound group C, 1.0%-2.0%.

While the present invention has been described with reference to certainpreferred embodiments, it is to be understood that the present inventionis not limited to such specific embodiments. Rather, it is theinventor's contention that the invention be understood and construed inits broadest meaning as reflected by the following claims. Thus, theseclaims are to be understood as incorporating not only the preferredembodiments described herein but all those other and further alterationsand modifications as would be apparent to those of ordinary skilled inthe art.

1. A non-aqueous electrolyte comprising organic solvents, electrolytesaline, and mixed additives, said mixed additives comprising: at least acompound of compound group A, at least a compound of compound group B,and at least a compound of compound group C, wherein: compound group Aare selected from inorganic saline including Li₂CO₃, Li₂SO₄, Li₂SO₃,LiNO₃; compound group B are selected from vinylene carbonate, andpropylene carbonate; and compound group C are selected from ES, PS,dimethyl sulfone, diethyl sulfone, and dimethyl sulphoxide.
 2. Thenon-aqueous electrolyte of claim 1, wherein the weight percentages ofthe compounds of the mixed additives in the non-aqueous electrolytecomprises are: compound group A: 0.1%-3.0%; compound group B: 0.5%-4.0%;and compound group C: 1.0%-5.0%.
 3. The non-aqueous electrolyte of claim1, wherein the weight percentages of the compounds of the mixedadditives in the non-aqueous electrolyte: compound group A: 0.3%-2.0%;compound group B: 1.0%-3.0%; and compound group C, 1.0%-3.0%.
 4. Thenon-aqueous electrolyte of claim 1, wherein the weight percentages ofthe compound of the mixed additives in the non-aqueous electrolyte:compound group A: 0.5%-1.0%; compound group B: 1.0%-3.0%; and compoundgroup C, 1.0%-2.0%.